it happens if the piano is not done with proper humidity control.
this is an image of sticky or what we call sluggish piano parts which cause the piano key stuck or slow response
due to high and low humidity occur, moist and damp hits inside the piano, then felts and clothes material parts will become slightly expand and increase the grip in every parts of the action then the key will become slow in repetition, stuck on and off, more serious, jammed and not functioning.
the action is the internal mechanism of the piano , to simplify, it is the heart of the piano. the action is made with many felt and cloth in every pivot points.
when such thing happen, an anti-friction treatment (overhaul) is the only way to fix the problem. the process is a time-consuming task and it will be pain-stacking if the piano is old.
this is a grand piano action part call the wippen, if you see clearly that every part of the movement joint are consist of a small piece of red felt supporting a pin to give a smooth movement